Unconformities are gaps in the rock record- representing times during.pdf
- 1. Unconformities are gaps in the rock record, representing times during which either no rocks were deposited, or existing rocks were eroded. The contact between older, eroded layers and younger, overlying layers is often irregular, so that unconformities are usually easy to recognize. Angular unconformities are contacts where younger rocks have been deposited horizontally on top of older rocks that had been tilted or folded. Across the resulting contact, the beds on one side of the angular unconformity are at an angle to the beds below it (i.e., not all the rock layers are parallel to one another). Cross-cutting relations states that any feature which cuts across a second feature is younger than the second feature. For example, if a fault cuts across rock layers, then the fault is younger than the rocks. If rocks are tilted or folded, the time of deformation is younger than the rocks. Unconformities and igneous intrusions can also cut across pre-existing layers. Figure 1 represents a vertical cross section, i.e., what you would see in a cliff face or road cut. The rock units and events are numbered according to stratigraphic principles. Layer #1 is the oldest, with #2 - #5 deposited successively on top of it. Then all five layers were tilted during time #6. Erosion followed, producing the angular unconformity #7. Layers #8 and #9 were deposited above the unconformity. Finallv, the igneous dike #10 was intruded, cutting across the older layers. Herrstrom, University of Illinois at Urbana-Champaign, 2018 Page 4 of 8 Because of the Principle of Included Fragments, we cannot use numerical dating to determine the ages of sedimentary rocks in the geologic cross-section. However, we can bracket the ages of these rocks using relative dating and the known ages of those rocks we can radiometrically date. Use the following geologic time scale to answer the last three questions regarding the ages of the sedimentary rocks in the 10. Over which geologic time periods did Beds 1 through 4 form? 11. Over which geologic time periods did Beds 5 through 9 form? 12. Bed 10 could not be older than the period.